docs: def convert_flow_coefficient(flow_coefficient, old_scale, new_scale):
      '''Convert from one flow coefficient scale to another; supports the `Kv`
      `Cv`, and `Av` scales.
      
      Other scales are `Qn` and `Cg`, but clear definitions have yet to be
      found.
      
      Parameters
      ----------
      flow_coefficient : float
          Value of the flow coefficient to be converted, expressed in the 
          original scale.
      old_scale : str
          String specifying the original scale; one of 'Av', 'Cv', or 'Kv', [-]
      new_scale : str
          String specifying the new scale; one of 'Av', 'Cv', or 'Kv', [-]
      
      Returns
      -------
      converted_flow_coefficient : float 
          Flow coefficient converted to the specified scale.
      
      Notes
      -----
      `Qn` is a scale based on a flow of air in units of L/minute as air travels
      through a valve and loses one bar of pressure (initially 7 bar absolute,
      to 6 bar absolute). No consistent conversion factors have been found and 
      those from theory do not match what have been found. Some uses of `Qn` use
      its flow rate as in normal (STP reference conditions) flow rate of air;
      others use something like the 7 bar absolute condition.
  
      Examples
      --------
      >>> convert_flow_coefficient(10, 'Kv', 'Av')
      0.0002776532068951358
      '''
      # Convert from `old_scale` to Kv
      if old_scale == 'Cv':
          Kv = Cv_to_Kv(flow_coefficient)
      elif old_scale == 'Kv':
          Kv = flow_coefficient
      elif old_scale == 'Av':
          Cv = flow_coefficient/((rho0/psi)**0.5*gallon/minute)
          Kv = Cv_to_Kv(Cv)
      else:
          raise NotImplementedError("%s scale is unsupported" %old_scale)
  
      if new_scale == 'Cv':
          ans = Kv_to_Cv(Kv)
      elif new_scale == 'Kv':
          ans = Kv
      elif new_scale == 'Av':
          Cv = Kv_to_Cv(Kv)
          ans = Cv*((rho0/psi)**0.5*gallon/minute)
      else:
          raise NotImplementedError("%s scale is unsupported" %old_scale)
  
      return ans